Technical Abstract:
Deep soil cores were collected from long-term experimental winter wheat fields at the Oklahoma State University Agricultural Research Station. The wheat fields are composed of four sets of sub-plots each having a fertilizer N application rate of 0, 40, 80, or 120 lbs/acre for the past 27 years. The soil cores were sampled at one-foot intervals from 0-10 feet deep. The nitrate concentration, chloride concentration, moisture content, and soil properties were measured for each sample. This research helps determine the maximum rate that fertilizer nitrogen (N) can be applied to winter wheat without causing significant contamination of nitrate to the groundwater. The nitrate concentration data indicate that the total mass of nitrate in the soil column does not increase significantly except within the 120 lbs/acre cores. Winter wheat production uses only approximately 55 lbs/acre. These results indicate that the fertilizer N application rate necessary for maximum crop yield is not high enough to contaminate the groundwater with nitrate, and current fertilizer N application rates of winter wheat seem to be safe. Two other Agricultural Research Stations (USDA ARS Grazing Lands Research Station and OSU Oklahoma Research Station) have been sampled in the same manner. The results of these two additional stations will be compared based on the laboratory results and the different environments of the stations. The comparison of the three stations may give insight into factors contributing to the loss of N and the minimization of nitrate leaching.